1. Field of the Invention
The present invention relates to a magneto-optic memory medium for recording, reproducing and erasing data using a laser, and a method for producing the same.
2. Description of the Related Art
As a magneto-optic memory material for use in a rewritable magneto-optic disc utilizing magneto-optical effects, rare earth transition metal films are actively studied and tried for practical use. However, the rare earth transition metal films involve problems in that the reliability is low due to a poor corrosion resistance and the level of a reproduced signal is low due to a small Kerr rotation angle. An increased level of the reproduced signal requires an enlarged Kerr rotation angle. In order to achieve a large apparent Kerr rotation angle, a reflection prevention structure including a magneto-optic memory layer and a dielectric layer (see, for example, IEEE Trans. Magn. MAG-16, p. 1194 (1980)), or a reflection film structure including a magneto-optic memory layer, a dielectric layer and a reflection layer are adopted to a magneto-optic memory medium. In order to increase the corrosion resistance and thus to raise the reliability, a passivation layer is adopted for covering a magneto-optic memory layer.
As is apparent from the above description, enlargement of the Kerr rotation angle is an important factor for the development of magneto-optic discs. Enlargement of a Kerr rotation angle requires raising the refractive index of a dielectric layer so as to utilize the enhancement effect to an utmost extent, which increases an apparent Kerr rotation angle (hereinafter, referred to simply as the "enhancement effect").
FIG. 4 shows a construction of an example of a conventional magneto-optic memory medium described in Japanese Laid-Open Patent Publication No. 59-110052. The magneto-optic memory medium includes a light transmitting substrate 1, a first dielectric layer 10 provided on the light transmitting substrate 1, a magneto-optic memory layer 4 provided on the first dielectric layer 10, a second dielectric layer 11 provided on the magneto-optic memory layer 4, and a reflection layer 6 provided on the second dielectric layer 11. The first dielectric layer 10 and the second dielectric layer 11 are formed of, for example, AlN.
Conventionally, in addition to AlN used in the example shown in FIG. 4, nitrides such as AlSiN and SiN and oxides such as Al.sub.2 O.sub.3 and SiO have been used for the dielectric layer. All of these materials have a refractive index of approximately 1.5 to 2.0, which is not sufficient to utilize the enhancement effect to an utmost extent. Oxides such as TiO.sub.2 and BaTiO.sub.3 have a high refractive index of approximately 2.4 to 2.6, but have not been used since free oxygen generated during the layer formation has an adverse effect on the protection of the magneto-optic memory layer, and thus lower the corrosion resistance and the reliability of the magneto-optic memory medium.